The present invention relates to compositions and methods for assaying homocysteine (Hcy) and thus related moieties, e.g., S-adenosylhomocysteine (SAH) or adenosine. More particularly, assay methods that employ, mutant SAH hydrolase having binding affinity for Hcy, SAH or adenosine but has attenuated catalytic activity, are provided. The modified enzymes and fusion proteins containing the modified enzymes are also provided.

Images(5)

Claims(20)

1. A method for assaying homocysteine (Hcy), S-adenosylhomocysteine (SAH) or adenosine in a sample, which method comprises:

a) contacting a sample containing or suspected of containing Hcy, SAH or adenosine with a mutant SAH hydrolase derived from a SAH hydrolase, wherein said SAH hydrolase is encoded by a nucleic acid comprising a nucleotide sequence selected from the group consisting of SEQ ID NO:185, SEQ ID NO:186, SEQ ID NO:187, and SEQ ID NO:188;

wherein said mutant SAH hydrolase has binding affinity for Hcy, SAH or adenosine but has attenuated catalytic activity, and wherein said binding affinity and/or said attenuated catalytic activity of said mutant SAH hydrolase is caused by a mutation in said mutant SAH hydrolase's catalytic site, its binding site for NAD+, NADH, Hcy, SAH, adenosine, or a combination thereof; and wherein the mutant SAH hydrolase: i) has a mutation in an amino acid residue that participates in catalysis or that is directly interacting with NAD+, NADH, Hcy, SAH or adenosine, or ii) has a mutation in an amino acid residue that is adjacent to an amino acid residue that participates in catalysis or that is directly interacting with NAD+, NADH, Hcy, SAH or adenosine, wherein the mutation in said mutant SAH hydrolase corresponding one or more amino acid positions selected from the group consisting of 53, 54, 57, 59, 80, 83, 121, 131, 134, 155, 157, 158, 159, 181, 190, 191, 214, 221, 226, 235, 240, 248, 263, 269, 285, 292, 301, 309, 322, 347, 351, 353, 361, 362, 379, 386, 388, 398, 401, 407, 409, 424, 425, 426, 427, 428, 429, 430, 431, and 432 SEQ ID NO:1 in said and

b) detecting binding between Hcy, SAH or adenosine with said mutant SAH hydrolase, whereby the presence or amount of Hcy, SAH or adenosine in said sample is assessed.

2. The method of claim 1, wherein the mutant SAH hydrolase has at least 50 fold higher binding affinity for Hcy, SAH or adenosine than a wild type SAH hydrolase from which said mutant SAH hydrolase is derived.

3. A method for assaying homocysteine (Hcy), S-adenosylhomocysteine (SAH) or adenosine in a sample, which method comprises:

a) contacting a sample containing or suspected of containing Hcy, SAH or adenosine with a mutant SAH hydrolase, wherein the mutant SAH hydrolase is derived from a human SAH hydrolase comprising the amino acid sequence set forth in SEQ ID NO:1, wherein said mutant SAH hydrolase has binding affinity for Hcy, SAH or adenosine but has attenuated catalytic activity, and wherein said binding affinity and/or said attenuated catalytic activity of said mutant SAH hydrolase is caused by a mutation in said mutant SAH hydrolase's catalytic site, its binding site for NAD+, NADH, Hcy, SAH, adenosine, or a combination thereof; and wherein the mutant SAH hydrolase: i) has a mutation in an amino acid residue that participates in catalysis or that is directly interacting with NAD+, NADH, Hcy, SAH or adenosine; or ii) has a mutation in an amino acid residue that is adjacent to an amino acid residue that participates in catalysis or that is directly interacting with NAD+, NADH, Hcy, SAH or adenosine, wherein the mutation in said mutant SAH hydrolase is at one or more amino acid positions selected from the group consisting of 38, 53, 54, 57, 59, 80, 83, 100, 121, 131, 134, 155, 157, 158, 159, 181, 190, 191, 214, 221, 226, 235, 240, 248, 263, 269, 285, 292, 301, 309, 322, 329, 347, 351, 353, 361, 362, 379, 386, 388, 398, 401, 407, 409, 420, 424, 425, 426, 427, 428, 429, 430, 431, and 432 in SEQ ID NO:1; and

b) detecting binding between Hey, SAH or adenosine with said mutant SAH hydrolase, whereby the presence or amount of Hcy, SAH or adenosine in said sample is assessed.

5. The method of claim 1, wherein prior to the contact between the sample and the mutant SAH hydrolase, oxidized or conjugated Hcy in the sample is converted into reduced Hcy by a reducing agent.

6. The method of claim 1, wherein prior to the contact between the sample and the mutant SAH hydrolase, the Hcy in the sample is converted into SAH.

7. The method of claim 5, further comprising a step of removing the reducing agent used to convert oxidized or conjugated Hcy into reduced Hcy prior to or concurrently with contacting the sample with the mutant SAH hydrolase, wherein the reducing agent is removed by chromatography.

8. The method of claim 1, wherein the sample is contacted with the mutant SAH hydrolase in the presence of a labeled SAH, thereby the amount of the labeled SAH, bound to the mutant SAH hydrolase inversely relates to the amount of SAH in the sample.

9. The method of claim 8, wherein the labeled SAH, is labeled with a fluorophore, an enzyme, or a protein.

12. The method of claim 9, wherein the fluorophore labeled SAH, is directly contacted by the mutant SAH hydrolase, and the resulting change of fluorescent polarization is measured for assessing the presence or amount of Hcy, SAH or adenosine in the sample.

13. The method of claim 9, wherein the enzyme labeled SAH, is directly contacted by the mutant SAH hydrolase, and the resulting change of enzyme activity is measured for assessing the presence or amount of Hcy, SAH or adenosine in the sample.

14. The method of claim 1, wherein the mutant SAH hydrolase is immobilized.

15. The method of claim 1, wherein the sample is a body fluid or a biological tissue.

16. The method of claim 3, wherein the mutant SAH hydrolase has at least 50 fold higher binding affinity for Hcy, SAH or adenosine than a wild type SAH hydrolase from which said mutant SAH hydrolase is derived.

17. The method of claim 6, wherein the sample is contacted with the mutant SAH hydrolase in the presence of a labeled SAH, thereby the amount of the labeled SAH, bound to the mutant SAH hydrolase inversely relates to the amount of SAH in the sample.

18. The method of claim 17, wherein the labeled SAH, is labeled with a fluorophore, an enzyme, or a protein.

20. The method of claim 3, wherein the mutant SAH hydrolase is immobilized.

Description

The present application is a continuation-in-part of U.S. patent application Ser. No. 09/457,205, filed Dec. 6, 1999, now abandoned, which is a continuation-in-part of U.S. patent application Ser. No. 09/347,878, filed Jul. 6, 1999, (now U.S. Pat. No. 6,376,210). The present application also claims priority benefit of U.S. Provisional Patent Application Ser. No. 60/301,895, filed Jun. 29, 2001. The disclosure of the above-referenced applications is incorporated by reference herein in their entirety.

TECHNICAL FIELD

The present invention relates to compositions and methods for assaying homocysteine (Hcy) and its related moieties, e.g., S-adenosylhomocysteine (SAH) or adenosine. More particularly, assay methods that employ, mutant SAH hydrolase having binding affinity for Hcy, SAH or adenosine but has attenuated catalytic activity, are provided. The modified enzymes and fusion proteins containing the modified enzymes are also provided.

BACKGROUND ART

Homocysteine (Hcy) is a thiol-containing amino acid formed from methionine during S-adenosylmethionine-dependent transmethylation reactions. Intracellular Hcy is remethylated to methionine, or is irreversibly catabolized in a series of reactions to form cysteine. Intracellular Hcy is exported into extracellular fluids such as blood and urine, and circulates mostly in oxidized form, and mainly bound to plasma protein (Refsum, et al., Annu. Rev. Medicine, 49:31–62 (1998)). The amount of Hcy in plasma and urine reflects the balance between Hcy production and utilization. This balance may be perturbed by clinical states characterized by genetic disorders of enzymes involved in Hcy transsulfuration and remethylation (e.g., cystathionine β-synthase and N5,10-methylenetetrahydrofolate reductase or dietary deficiency of vitamins (e.g., vitamin B6, B12 and folate) involved in Hcy metabolism (Baual, et al., Cleveland Clinic Journal of Medicine, 64:543–549 (1997)). In addition, plasma Hcy levels may also be perturbed by some medications such as anti-folate drugs (e.g., methotrexate) used for treatments of cancer or arthritis (Foody, et al., Clinician Reviews, 8:203–210 (1998)).

Severe cases of homocysteinemia are caused by homozygous defects in genes encoding for enzymes involved in Hcy metabolisms. In such cases, a defect in an enzyme involved in either Hcy remethylation or transsulfuration leads to as much as 50-fold elevations of Hcy in the blood and urine. The classic form of such a disorder, congenital homocysteinemia (Hcyemia), is caused by homozygous defects in the gene encoding cystathionine β-synthase (CBS). These individuals suffer from thromboembolic complications at an early age, which result in stroke, myocardial infarction, renovascular hypertension, intermittent claudication, mesenteric ischemic, and pulmonary embolism. Such patients may also exhibit mental retardation and other abnormalities resembling ectopia lentis and skeletal deformities (Perry T., Homocysteine: Selected aspects in Nyham W. L. ed. Heritable disorders of amino acid metabolism. New York, John Wiley & Sons, pp. 419–451 (1974)). It is also known that elevated Hcy levels in pregnant women is related to birth defects of children with neurotube closures (Scott, et al., “The etiology of neural tube defects” in Graham, I., Refsum, H., Rosenberg, I. H., and Ureland P. M. ed. “Homocysteine metabolism: from basic science to clinical medicine” Kluwer Academic Publishers, Boston, pp. 133–136 (1995)). Thus, the diagnostic utility of Hcy determinations has been well documented in these clinical conditions.

It has been demonstrated that even mild or moderately elevated levels of Hcy also increase the risk of atherosclerosis of the coronary, cerebral and peripheral arteries and cardiovascular disease (Boushey, et al., JAMA, 274:1049–1057 (1995)). The prevalence of Hcyemia was shown to be 42%, 28%, and 30% among patients with cerebral vascular disease, peripheral vascular disease and cardiovascular disease, respectively (Moghadasian, et al., Arch. Intern. Med., 157:2299–2307 (1997)). A meta-analysis of 27 clinical studies calculated that each increase of 5 μM in Hcy level increases the risk for coronary artery disease by 60% in men and by 80% in women, which is equivalent to an increase of 20 mg/dl−1 (0.5 mmol/dl−1) in plasma cholesterol, suggesting that Hcy, as a risk factor, is as strong as cholesterol in the general population. Results from these clinical studies concluded that hyperhomocysteinemia is an emerging new independent risk factor for cardiovascular disease, and may be accountable for half of all cardiovascular patients who do not have any of the established cardiovascular risk factors (e.g., hypertension, hypercholesterolemia, cigarette smoking, diabetes mellitus, marked obesity and physical inactivity).

Most of the methods require sophisticated chromatographic techniques such as HPLC, capillary gas chromatography, or mass spectrometry (GC/MS) to directly or indirectly (e.g., enzymatic conversion of Hcy to SAH (S-adenosylhomocysteine) by SAH hydrolase followed by HPLC or TLC separation) measure Hcy. Radioenzymatic conversion of Hcy to radiolabeled SAH by SAH hydrolase prior to TLC separation has also been used. In these assays, chromatographic separation, which is often time-consuming and cumbersome to perform, is a common key step of these methods. More particularly, these methods require highly specialized and sophisticated equipment and well-trained analytic specialists. The use of such equipment is generally not well-accepted in routine clinical laboratory practice.

Immunoassays for Hcy that use a monoclonal antibody against SAH (Araki, et al., J Chromatog., 422:43–52 (1987)) are also known. These assays are based upon conversion of Hcy to SAH, which is then detected by a monoclonal antibody. Monoclonal antibody against albumin-bound Hcy has been developed for determination of albumin-bound Hcy (Stabler, et al., J. Clin. Invest., 81:466–474 (1988)), which is the major fraction of total plasma Hcy. Other immunological protocols are also available (see, e.g., U.S. Pat. Nos. 5,631,127, 5,827,645, 5,958,717, 6,063,581 and 5,885,767). Though immunoassays avoid a time-consuming chromatographic separation step and are amenable to automation, production of monoclonal antibody is expensive, somewhat unpredictable, and often requires secondary or even tertiary antibodies for detection.

It is an object herein to provide assays for quantifying and/or detecting homocysteine in body fluids and body tissues.

DISCLOSURE OF THE INVENTION

Assays that employ mutant SAH hydrolase having binding affinity for Hcy, SAH or adenosine but has attenuated catalytic activity are provided. These methods are designated substrate trapping methods; and the modified SAH hydrolases, are designated as “substrate trapping SAH hydrolases.” The substrate trapping SAH hydrolases and methods for preparing them are also provided. The assays can be used for prognostic, diagnostic, drug screening or treatment monitoring purposes.

The assays readily can be automated. In addition, the assays can be adapted for use in point of care systems and in home test kits. For example, blood test point of care systems can be adapted for measuring homocysteine levels using the mutant SAH hydrolases provided herein. Home test kits may also be adapted for use with the methods and mutant SAH hydrolases provided herein.

In one aspect, the present invention is directed to a method for assaying homocysteine (Hcy), S-adenosylhomocysteine (SAH) or adenosine in a sample, which method comprises: a) contacting a sample containing or suspected of containing Hcy, SAH or adenosine with a mutant SAH hydrolase, wherein said mutant SAH hydrolase has binding affinity for Hcy, SAH or adenosine but has attenuated catalytic activity, and said binding affinity and/or said attenuated catalytic activity of said SAH hydrolase is caused by a mutation in said mutant SAH hydrolase's catalytic site, its binding site for NAD+, NADH, Hcy, SAH or adenosine, or a combination thereof; and b) detecting binding between Hcy, SAH or adenosine with said mutant SAH hydrolase, whereby the presence or amount of Hcy, SAH or adenosine in said sample is assessed.

Any suitable mutant SAH hydrolases can be used in the present methods. In one example, the mutant SAH hydrolase used in the method has a mutation in an amino acid residue that is directly involved in the SAH hydrolase's catalytic activity, its binding with NAD+, NADH, Hcy, SAH or adenosine. In another example, the mutant SAH hydrolase used in the method has a mutation in an amino acid residue that is adjacent to an amino acid residue that is directly involved in the SAH hydrolase's catalytic activity, its binding with NAD+, NADH, Hcy, SAH or adenosine.

The mutant SAH hydrolase can have enhanced binding affinity for Hcy, SAH or adenosine than a wild type SAH hydrolase from which said mutant SAH hydrolase is derived. Preferably, the mutant SAH hydrolase has at least 10, 20, 30, 40, 50, 60, 70, 80, 90 or 100 fold higher binding affinity for Hcy, SAH or adenosine than a wild type SAH hydrolase from which said mutant SAH hydrolase is derived.

The mutant SAH hydrolase can be derived from any suitable sources. For example, the mutant SAH hydrolase can be derived from a mammalian SAH hydrolase, e.g., derived from a human SAH hydrolase.

Prior to the contact between the sample and the mutant SAH hydrolase, oxidized or conjugated Hcy in the sample can be converted into reduced Hcy. Similarly, prior to the contact between the sample and the mutant SAH hydrolase, the Hcy in the sample can be converted into SAH. Preferably, oxidized or conjugated Hcy in the sample is converted into reduced Hcy and then the reduced Hcy is converted into SAH.

Hcy in the sample can be converted into SAH by any suitable method, e.g., by a wild-type SAH hydrolase and access adenosine. Preferably, the access adenosine in the sample is removed by adenosine deaminase while the wild-type SAH hydrolase is inhibited. Any suitable SAH hydrolase inhibitors can be used, e.g., neplanocin A or aristeromycin.

The present method can further comprise a step of removing the reducing agent used to convert oxidized or conjugated Hcy into reduced Hcy prior to or concurrently with contacting the sample with the mutant SAH hydrolase. The reducing agent can be removed by any suitable methods such as chromatography. Exemplary chromatography includes column, paper, thin layer and batch chromatography. Preferably, the reducing agent is removed by batch chromatography. The removal of the reducing agent can have certain benefits such as increasing assay sensitivity. The degree of the removal is affected by a number of factors including the reducing agent used, the removal methods used and the objective of the removal. Although the reducing agent may be substantially removed, it is often not necessary to do so; for a 10–50 percent reduction of the reducing agent can bring significant benefits to the assay.

An indicator dye can be used in the present method for various reasons, e.g., for the ease of monitoring sample and reagent addition and transfer. The present method can further comprise a step of removing the indicator dye prior to or concurrently with contacting the sample with the mutant SAH hydrolase. The indicator dye can be removed by any suitable methods such as chromatography. Exemplary chromatography includes column, paper, thin layer and batch chromatography. Preferably, the indicator dye is removed by batch chromatography. The degree of the removal is affected by a number of factors including the indicator dye used, the removal methods used and the objective of the removal. Although the reducing agent may be substantially removed, it is often not necessary to do so; for a 10–70 percent reduction of the indicator dye can bring significant benefits to the assay.

In a specific embodiment, the reducing agent and the indicator dye are removed by the same method, e.g., chromatography. Preferably, the reducing agent and the indicator dye are removed by a batch chromatography.

The present method can be conducted in any suitable format, e.g., in a competitive or sandwich format, in solution or on a solid support. In a specific embodiment, the SAH is contacted with the mutant SAH hydrolase in the presence of a labeled SAH or a derivative or an analogue thereof, thereby the amount of the mutant SAH hydrolase bound to the labeled SAH inversely relates to the amount of SAH in the sample. Any suitable labels can be used such as chemical, radioactive, enzymatic, fluorescent or luminescent label. Preferably, the labeled SAH or a derivative or an analogue thereof is fluorescently, enzymatically or proteinaceously labeled. For example, the fluorescently labeled SAH can be fluorecin-SAH conjugate or Rocamin-SAH conjugate, said fluorecin or Rocamin being linked to said SAH or a derivative or an analogue thereof by a linker of 1–15 carbon atom length; the enzymatically labeled SAH derivative can be Glucose-6-phosphate dehydrogenase (G-6-PDH-SAH) conjugate, alkaline phosphatase-SAH conjugate, or malate dehydrolase-SAH conjugate, said G-6-PDH, alkaline phosphatase or malate dehydrolase being linked to said SAH or a derivative or an analogue thereof by a linker of 1–15 carbon atom length; and the proteinaceously labeled SAH derivative can be bovine albumin-SAH conjugate, said bovine albumin being linked to said SAH or a derivative or an analogue thereof by a linker of 1–15 carbon atom length.

In another specific embodiment, the mutant SAH hydrolase is a labeled mutant SAH hydrolase. Any suitable labels can be used such as chemical, radioactive, enzymatic, fluorescent or luminescent label. Preferably, the labeled mutant SAH hydrolase is a fluorescently, enzymatically, biotin or streptavidin labeled mutant SAH hydrolase. For example, the biotin labeled mutant SAH hydrolase is detected by a streptavidin labeled enzyme; the streptavidin labeled enzyme is a streptavidin labeled horse radish phosphatase (HRP).

The labeled SAH or a derivative or an analogue thereof, or the labeled mutant SAH hydrolase can be immobilized on a surface suitable for conducting an assay for Hcy or its related moieties. For example, the bovine albumin-SAH conjugate can be immobilized.

In a specific assay format, the fluorescently labeled SAH or a derivative or an analogue thereof is directly contacted by the mutant SAH hydrolase, and the resulting change of fluorescent polarization is measured for assessing Hcy, SAH or adenosine. In another specific assay format, the enzymatically labeled SAH or a derivative or an analogue thereof is directly contacted by the mutant SAH hydrolase, and the resulting change of enzyme activity is measured for assessing Hcy, SAH or adenosine.

The present method can be used to assay any sample, e.g., a body fluid or a biological tissue. Exemplary body fluids include urine, blood, plasma, serum, saliva, semen, stool, sputum, cerebral spinal fluid, tears, mucus and amniotic fluid. Preferably, the body fluid to be assayed is blood. The blood sample can be assayed directly or be treated before assaying. For example, the blood sample can be further separated into a plasma or serum fraction.

The present method can be used alone, or can be used in combination with other related assays. For example, the present method can further comprise a step of detecting cholesterol and/or folic acid in the sample, whether sequentially or simultaneously.

In another aspect, the present invention is directed to a combination, which combination comprises: a) a mutant SAH hydrolase that has binding affinity for Hcy, SAH or adenosine but has attenuated catalytic activity, wherein said binding affinity and/or said attenuated catalytic activity of said SAH hydrolase is caused by a mutation in said mutant SAH hydrolase's catalytic site, its binding site for NAD+, NADH, Hcy, SAH or adenosine, or a combination thereof; and b) reagents for detecting binding between Hcy, SAH or adenosine and said SAH hydrolase. The combination can further comprise a reagent for detecting cholesterol and/or folic acid.

A kit comprising the above combination is also provided. The kit can further comprise instructions for assaying Hcy, SAH or adenosine in a sample.

In still another aspect, the present invention is directed to an article of manufacture, which article of manufacture comprises: a) packaging material; b) a mutant SAH hydrolase that has binding affinity for Hcy, SAH or adenosine but has attenuated catalytic activity, wherein said binding affinity and/or said attenuated catalytic activity of said SAH hydrolase is caused by a mutation in said mutant SAH hydrolase's catalytic site, its binding site for NAD+, NADH, Hcy, SAH or adenosine, or a combination thereof; and c) a label indicating that the mutant SAH hydrolase and the means for use in assaying Hcy, SAH or adenosine in a sample.

The isolated nucleic acid fragment can be in any suitable forms, e.g., DNA, RNA, PNA, etc., or a combination thereof. A plasmid comprising the above isolated nucleic acid fragment is also provided. A cell comprising the above plasmid is also provided. Any suitable cells can be used. For example, the cell can be a bacterial cell, a yeast cell, a fungal cell, a plant cell, an insect cell or an animal cell. A method for producing a mutant SAH hydrolase is also provided, which method comprises growing the above cell under conditions whereby the mutant SAH hydrolase is expressed by the cell, and recovering the expressed mutant SAH hydrolase.

In yet another aspect, the present invention is directed to a conjugate, which conjugate comprises: a) a mutant SAH hydrolase that has binding affinity for Hcy, SAH or adenosine but has attenuated catalytic activity, wherein said binding affinity and/or said attenuated catalytic activity of said SAH hydrolase is caused by a mutation in said mutant SAH hydrolase's catalytic site, its binding site for NAD+, NADH, Hcy, SAH or adenosine, or a combination thereof; and b) a facilitating agent linked to the mutant SAH hydrolase directly or via a linker, wherein the agent facilitates: i) affinity isolation or purification of a conjugate; ii) attachment of a conjugate to a surface; or iii) detection of a conjugate. The conjugate can be a fusion protein. Alternatively, the mutant SAH hydrolase and the facilitating agent can be linked by other means.

For clarity of disclosure, and not by way of limitation, the detailed description of the invention is divided into the subsections that follow.

A. DEFINITIONS

Unless defined otherwise, all technical and scientific terms used herein have the same meaning as is commonly understood by one of ordinary skill in the art to which this invention belongs. All patents, applications, published applications and other publications referred to herein are incorporated by reference in their entirety. If a definition set forth in this section is contrary to or otherwise inconsistent with a definition set forth in the patents, applications, published applications and other publications that are herein incorporated by reference, the definition set forth in this section prevails over the definition that is incorporated herein by reference.

As used herein, “a” or “an” means “at least one” or “one or more.”

As used herein, “homocysteine (Hcy)” refers to a compound with the following molecular formula: HSCH2CH2CH(NH2)COOH. Biologically, Hcy is produced by demethylation of methionine and is an intermediate in the biosynthesis of cysteine from methionine. The term “Hcy” encompasses free Hcy (in the reduced form) and conjugated Hcy (in the oxidized form). Hcy can conjugate with proteins, peptides, itself or other thiols through disulfide bond.

As used herein, “SAH hydrolase” refers to an ubiquitous eukaryotic enzyme, which is also found in some prokaryotes, which catalyzes hydrolysis of SAH to adenosine (Ado) and Hcy. SAH hydrolase also catalyzes the formation of SAH from Ado and Hcy. The co-enzyme of SAH hydrolase is NAD+/NADH. SAH hydrolase may have several catalytic activities. In the hydrolytic direction, the first step involves oxidation of the 3′-hydroxyl group of SAH (3′-oxidative activity) by enzyme-bound NAD+ (E-NAD+), followed by β-elimination of L-Hcy to give 3′-keto-4′,5′-didehydro-5′-deoxy-Ado. Michael addition of water to the 5′-position to this tightly bound intermediate (5′-hydrolytic activity) affords 3′-keto-Ado, which is then reduced by enzyme-bound NADH (E-NADH) to Ado (3′-reduction activity). It is intended to encompass SAH hydrolase with conservative amino acid substitutions that do not substantially alter its activity.

As used herein, “mutant SAH hydrolase, wherein said mutant SAH hydrolase has binding affinity for Hcy, SAH or adenosine but has attenuated catalytic activity” refers to a mutant form of SAH hydrolase that retains sufficient binding affinity for Hcy, SAH or adenosine to be detected in the process or method, particularly assay, of interest. Typically this is at least about 10%, preferably at least about 50% binding affinity for Hcy, SAH or adenosine, compared to its wildtype counterpart SAH hydrolase. Preferably, such mutant SAH hydrolase retains 60%, 70%, 80%, 90%, 100% binding affinity for Hcy, SAH or adenosine compared to its wildtype counterpart Hcy, SAH or adenosine, or has a higher binding affinity than its wildtype counterpart Hcy, SAH or adenosine. Such mutant Hcy, SAH or adenosine can be herein referred to as a “substrate trapping Hcy, SAH or adenosine,” i.e., a molecule that specifically binds to Hcy, SAH or adenosine, but does not catalyze conversion therebetween.

As used herein, a conjugate refers to the compounds provided herein that include one or more mutant analyte-binding enzymes, e.g., mutant SAH hydrolase, and one or more facilitating agents. These conjugates include those produced by recombinant means as fusion proteins, those produced by chemical means, such as by chemical coupling, through, for example, coupling to sulfhydryl groups, and those produced by any other method whereby at least one mutant analyte-binding enzyme is linked, directly or indirectly via linker(s) to a facilitating agent.

As used herein, a facilitating agent is any moiety, such as a protein or effective portion thereof, that promotes or facilitates, for example, preferably:

i) affinity isolation or purification of the conjugate;

ii) attachment of the conjugate to a surface; or

iii) detection of the conjugate or complexes containing the conjugate.

As used herein the term “assessing” is intended to include quantitative and qualitative determination in the sense of obtaining an absolute value for the amount or concentration of the analyte, e.g., a homocysteine co-substrate, present in the sample, and also of obtaining an index, ratio, percentage, visual or other value indicative of the level of analyte in the sample. Assessment may be direct or indirect and the chemical species actually detected need not of course be the analyte itself but may for example be a derivative thereof or some further substance.

As used herein, “attenuated catalytic activity” refers to a mutant SAH hydrolase that retains sufficiently reduced catalytic activity to be useful in the present method. The precise reduction in catalytic activity for use in the assays can be empirically determined for each assay. Typically, the enzyme will retain less than about 50% of one of its catalytic activities or less than 50% of its overall catalytic activities compared to its wildtype counterpart. Preferably, a mutant SAH hydrolase retains less than 40%, 30%, 20%, 10%, 1%, 0.1%, or 0.01% of one of its catalytic activities or its overall catalytic activities compared to its wildtype counterpart. More preferably, a mutant SAH hydrolase lacks detectable level of one of its catalytic activities or its overall catalytic activities compared to its wildtype counterpart. In instances in which catalytic activity is retained and/or a further reduction thereof is desired, the contacting step can be effected in the presence of a catalysis inhibitor. Such inhibitors, include, but are not limited to, heavy metals, chelators or other agents that bind to a co-factor required for catalysis, but not for binding, and other such agents.

As used herein, “SAH hydrolase catalysis inhibitor” refers to an agent that inhibits one or all of SAH hydrolase catalytic activities, e.g., 3′-oxidative activity, 5′-hydrolytic activity, or 3′-reduction activity, while not affecting SAH hydrolase's binding affinity for Hcy and/or SAH.

As used herein, “adenosine deaminase” refers to an enzyme that catalyzes the deamination of adenosine to form inosine. It is intended to encompass adenosine deaminase with conservative amino acid substitutions that do not substantially alter its activity.

As used herein, “folate species” refers to folate or folic acid, which is chemically N-[4-[[2-amino-1,4-dihydro-4-oxo-6-pteridinyl)methyl]amino]benzxoyl]-L-glutamic acid, or a derivative thereof. Examples of folate derivatives include, but are not limited to, dihydrofolate, tetrahydrofolate, 5,-methyl-tetrahydrofolate and 5,10-methylene tetrahydrofolate.

As used herein, “serum” refers to the fluid portion of the blood obtained after removal of the fibrin clot and blood cells, distinguished from the plasma in circulating blood.

As used herein, “plasma” refers to the fluid, noncellular portion of the blood, distinguished from the serum obtained after coagulation.

As used herein, “substantially pure” means sufficiently homogeneous to appear free of readily detectable impurities as determined by standard methods of analysis, such as thin layer chromatography (TLC), gel electrophoresis and high performance liquid chromatography (HPLC), used by those of skill in the art to assess such purity, or sufficiently pure such that further purification would not detectably alter the physical and chemical properties, such as enzymatic and biological activities, of the substance. Methods for purification of the compounds to produce substantially chemically pure compounds are known to those of skill in the art. A substantially chemically pure compound may, however, be a mixture of stereoisomers or isomers. In such instances, further purification might increase the specific activity of the compound.

As used herein, “antibody” includes antibody fragments, such as Fab fragments, which are composed of a light chain and the variable region of a heavy chain.

As used herein, “production by recombinant means” refers to production methods that use recombinant nucleic acid methods that rely on well known methods of molecular biology for expressing proteins encoded by cloned nucleic acids.

As used herein, “complementary” when referring to two nucleic acid molecules, means that the two sequences of nucleotides are capable of hybridizing, preferably with less than 25%, more preferably with less than 15%, even more preferably with less than 5%, most preferably with no mismatches between opposed nucleotides. Preferably the two molecules will hybridize under conditions of high stringency.

As used herein: “stringency of hybridization” in determining percentage mismatch is as follows:

It is understood that equivalent stringencies may be achieved using alternative buffers, salts and temperatures.

The term “substantially” identical or homologous or similar varies with the context as understood by those skilled in the relevant art and generally means at least 70%, preferably means at least 80%, more preferably at least 90%, and most preferably at least 95% identity.

As used herein, a “composition” refers to any mixture of two or more products or compounds. It may be a solution, a suspension, liquid, powder, a paste, aqueous, non-aqueous or any combination thereof.

As used herein, a “combination” refers to any association between two or among more items.

As used herein, “fluid” refers to any composition that can flow. Fluids thus encompass compositions that are in the form of semi-solids, pastes, solutions, aqueous mixtures, gels, lotions, creams and other such compositions.

As used herein, “vector (or plasmid)” refers to discrete elements that are used to introduce heterologous DNA into cells for either expression or replication thereof. Selection and use of such vehicles are well known within the skill of the artisan. An expression vector includes vectors capable of expressing DNA's that are operatively linked with regulatory sequences, such as promoter regions, that are capable of effecting expression of such DNA fragments. Thus, an expression vector refers to a recombinant DNA or RNA construct, such as a plasmid, a phage, recombinant virus or other vector that, upon introduction into an appropriate host cell, results in expression of the cloned DNA. Appropriate expression vectors are well known to those of skill in the art and include those that are replicable in eukaryotic cells and/or prokaryotic cells and those that remain episomal or those which integrate into the host cell genome.

As used herein, “a promoter region or promoter element” refers to a segment of DNA or RNA that controls transcription of the DNA or RNA to which it is operatively linked. The promoter region includes specific sequences that are sufficient for RNA polymerase recognition, binding and transcription initiation. This portion of the promoter region is referred to as the promoter. In addition, the promoter region includes sequences that modulate this recognition, binding and transcription initiation activity of RNA polymerase. These sequences may be cis acting or may be responsive to trans acting factors. Promoters, depending upon the nature of the regulation, may be constitutive or regulated. Exemplary promoters contemplated for use in prokaryotes include the bacteriophage T7 and T3 promoters, and the like.

As used herein, “operatively linked or operationally associated” refers to the functional relationship of DNA with regulatory and effector sequences of nucleotides, such as promoters, enhancers, transcriptional and translational stop sites, and other signal sequences. For example, operative linkage of DNA to a promoter refers to the physical and functional relationship between the DNA and the promoter such that the transcription of such DNA is initiated from the promoter by an RNA polymerase that specifically recognizes, binds to and transcribes the DNA. In order to optimize expression and/or in vitro transcription, it may be necessary to remove, add or alter 5′ untranslated portions of the clones to eliminate extra, potential inappropriate alternative translation initiation (i.e., start) codons or other sequences that may interfere with or reduce expression, either at the level of transcription or translation. Alternatively, consensus ribosome binding sites (see, e.g., Kozak, J. Biol. Chem., 266:19867–19870 (1991)) can be inserted immediately 5′ of the start codon and may enhance expression. The desirability of (or need for) such modification may be empirically determined.

As used herein, “sample” refers to anything which may contain an analyte for which an analyte assay is desired. The sample may be a biological sample, such as a biological fluid or a biological tissue. Examples of biological fluids include urine, blood, plasma, serum, saliva, semen, stool, sputum, cerebral spinal fluid, tears, mucus, amniotic fluid or the like. Biological tissues are aggregates of cells, usually of a particular kind together with their intercellular substance that form one of the structural materials of a human, animal, plant, bacterial, fungal or viral structure, including connective, epithelium, muscle and nerve tissues. Examples of biological tissues also include organs, tumors, lymph nodes, arteries and individual cell(s).

As used herein, the abbreviations for any protective groups, amino acids and other compounds, are, unless indicated otherwise, in accord with their common usage, recognized abbreviations, or the IUPAC-IUB Commission on Biochemical Nomenclature (see, (1972) Biochem. 11:1726).

As used herein, “protein binding sequence” refers to a protein or peptide sequence that is capable of specific binding to other protein or peptide sequences generally, to a set of protein or peptide sequences or to a particular protein or peptide sequence.

As used herein, “epitope tag” refers to a short stretch of amino acid residues corresponding to an epitope to facilitate subsequent biochemical and immunological analysis of the “epitope tagged” protein or peptide. “Epitope tagging” is achieved by appending the sequence of the “epitope tag” to the protein-encoding sequence in an appropriate expression vector. “Epitope tagged” proteins can be affinity purified using highly specific antibodies raised against the tags.

As used herein, “Protein A or Protein G” refers to proteins that can bind to Fc region of most IgG isotypes. Protein A or Protein G are typically found in the cell wall of some strains of staphylococci. It is intended to encompass Protein A or Protein G with conservative amino acid substitutions that do not substantially alter its activity.

As used herein, “nucleotide binding sequence” refers to a protein or peptide sequence that is capable of specific binding to nucleotide sequences generally, to a set of nucleotide sequences or to a particular nucleotide sequence.

As used herein, “lipid binding sequence” refers to a protein or peptide sequence that is capable of specific binding to lipids generally, to a set of lipids or to a particular lipid.

As used herein, “polysaccharide binding sequence” refers to a protein or peptide sequence that is capable of specific binding to polysaccharides generally, to a set of polysaccharides or to a particular polysaccharide.

As used herein, “metal binding sequence” refers to a protein or peptide sequence that is capable of specific binding to metal ions generally, to a set of metal ions or to a particular metal ion.

As used herein, “alkaline phosphatases” refers to a family of functionally related enzymes named after the tissues in which they predominately appear. Alkaline phosphatases carry out hydrolase/transferase reactions on phosphate-containing substrates at a high pH optimum. It is intended to encompass alkaline phosphatases with conservative amino acid substitutions that do not substantially alter its activity.

As used herein, “glutathione S-transferase” refers to a ubiquitous family of enzymes with dual substrate specificities that perform important biochemical functions of xenobiotic biotransformation and detoxification, drug metabolism, and protection of tissues against peroxidative damage. The basic reaction catalyzed by glutathione S-transferase is the conjugation of an electrophile with reduced glutathione (GSH) and results in either activation or deactivation/detoxification of the chemical. It is intended to encompass a glutathione S-transferase with conservative amino acid substitutions that do not substantially alter its activity.

As used herein, “disease or disorder” refers to a pathological condition in an organism resulting from, e.g., infection or genetic defect, and characterized by identifiable symptoms.

B. METHODS FOR ASSAYING HCY, SAH OR ADENOSINE

In one aspect, the present invention is directed to a method for assaying homocysteine (Hcy), S-adenosylhomocysteine (SAH) or adenosine in a sample, which method comprises: a) contacting a sample containing or suspected of containing Hcy, SAH or adenosine with a mutant SAH hydrolase, wherein said mutant SAH hydrolase has binding affinity for Hcy, SAH or adenosine but has attenuated catalytic activity, and said binding affinity and/or said attenuated catalytic activity of said SAH hydrolase is caused by a mutation in said mutant SAH hydrolase's catalytic site, its binding site for NAD+, NADH, Hcy, SAH or adenosine, or a combination thereof; and b) detecting binding between Hcy, SAH or adenosine with said mutant SAH hydrolase, whereby the presence or amount of Hcy, SAH or adenosine in said sample is assessed.

Nucleic Acids Encoding SAH Hydrolase

Nucleic acids encoding SAH hydrolase can be obtained by methods known in the art. Additional nucleic acid molecules encoding such enzymes are known and the molecules or sequences thereof are publicly available. If the molecules are available they can be used; alternatively the known sequences can be used to obtain clones from selected or desired sources. For example, the nucleic acid sequences of SAH hydrolases can be used in isolating nucleic acids encoding SAH hydrolases from natural sources. Alternatively, nucleic acids encoding SAH hydrolases can be obtained by chemical synthesis according to the known sequences.

Once nucleic acids encoding SAH hydrolases are obtained, these nucleic acids can be mutagenized and screened and/or selected for mutant SAH hydrolase having binding affinity for Hcy, SAH or adenosine but having attenuated catalytic activity. Insertion, deletion, or point mutation(s) can be introduced into nucleic acids encoding SAH hydrolases according to methods known to those of skill in the art. Information regarding the structural-functional relationship of the SAH hydrolases can be used in the mutagenesis and selection of mutant SAH hydrolase having binding affinity for Hcy, SAH or adenosine but having attenuated catalytic activity.

In one example, the mutant SAH hydrolase used in the method has a mutation in an amino acid residue that is directly involved in the SAH hydrolase's catalytic activity, its binding with NAD+, NADH, Hcy, SAH or adenosine. In another example, the mutant SAH hydrolase used in the method has a mutation in an amino acid residue that is adjacent to an amino acid residue that is directly involved in the SAH hydrolase's catalytic activity, its binding with NAD+, NADH, Hcy, SAH or adenosine. Information on the SAH hydrolase's catalytic domain, various binding domains including the NAD binding domain and conserved amino acid residues are generally known and can be used in the designing of a suitable mutant SAH hydrolase (See e.g., Turner et al., Nat. Struct. Biol., 5(5):369–76 (1998) entitled “Structure determination of selenomethionyl S-adenosylhomocysteine hydrolase using data at a single wavelength;” Yin et al., Biomedical Chemistry: Applying Chemical Principles to the Understading and Treatetment of Diesease (Ed. Torrence), Chapter 2, Mechanism-based S-adenosylhomocysteine hydrolase inhibitors in the saerch for broad-spectrum antiviral agents), John Wiley & Sons, Inc. (2000); Hu et al., Biochemistry, 38(26):8323–33 (1999) entitled “Crystal structure of S-adenosylhomocysteine hydrolase from rat liver;” Creedon et al., J. Biol. Chem., 269(23):16364–70 (1994) entitled “Plasmodium falciparum S-adenosylhomocysteine hydrolase. cDNA identification, predicted protein sequence, and expression in Escherichia coli.;” and Henderson et al., Mol. Biochem. Parasitol., 53(1-2): 169–83 (1992) entitled “Cloning of the gene encoding Leishmania donovani S-adenosylhomocysteine hydrolase, a potential target for antiparasitic chemotherapy.”

Once a mutant SAH hydrolase with desired properties, i.e., substantially retaining binding affinity for Hcy, SAH or adenosine but having attenuated catalytic activity, is identified, such mutant SAH hydrolase can be produced by any methods known in the art including recombinant expression, chemical synthesis or a combination thereof. Preferably, the mutant SAH hydrolase is obtained by recombinant expression.

SAH hydrolase from mammalian sources are homotetramer of approximate molecular weight of 180–190 KD. The enzyme contains 4 molecules of tightly-bound NAD+ as a co-enzyme. The catalytic mechanism of the enzyme in the hydrolytic direction includes two consecutive reactions, i.e., the 3′-oxidation of the substrate to 3′-keto in concomitant with the reduction of the enzyme-bound NAD+ to NADH, and followed by the 5′-hydrolysis to release the reaction products Hcy and Ado (Refsum, et al., Clin. Chem., 31:624–628 (1985)). The C-terminal regions of all known SAH hydrolase are extremely conserved and contain essential amino acid residues to the enzyme catalysis. The crystal structure of human SAH hydrolase in complex with a substrate analog inhibitor was recently determined. This x-ray structure of SAH hydrolase indicates that at least twenty amino acid residues are directly or indirectly interacting with the substrate analog inhibitor and co-enzyme NAD+. Mutations of those amino acid residues that are involved directly or indirectly in the substrate binding and catalysis can readily be made by site-directed mutagenesis, and the sequence of the resulting mutant enzyme can be confirmed by comparing the mutant SAH hydrolase DNA sequence with the sequence of the wild type enzyme to ensure no other mutations are introduced to the specific mutant enzyme.

Provided herein is a substantially purified mutant SAH hydrolase that substantially retains its binding affinity or has enhanced binding affinity for Hcy, SAH or adenosine but has attenuated catalytic activity.

In one specific embodiment, the attenuated catalytic activity of the mutant SAH hydrolase is caused by mutation(s) in the mutant SAH hydrolase's binding site for NAD+, or mutation(s) in the mutant SAH hydrolase's catalytic site or a combination thereof.

In another specific embodiment, the mutant SAH hydrolase has attenuated 5′-hydrolytic activity but substantially retains its 3′-oxidative activity.

In still another specific embodiment, the mutant SAH hydrolase irreversibly binds SAH.

In yet another specific embodiment, the mutant SAH hydrolase has a Km for SAH that is about or less than 10.0 μM. Preferably, the mutant SAH hydrolase has a Km for SAH that is about 1.0 μM or less than 1.0 μM.

In yet another specific embodiment, the mutant SAH hydrolase has a Kcat for SAH that is about or less than 0.1 S−1.

In yet another specific embodiment, the mutant SAH hydrolase has one or more insertion, deletion, or point mutation(s). Preferably, the mutant SAH hydrolase is derived from the sequence of amino acids set forth in SEQ ID NO:1 or encoded by the sequence of nucleotides set forth in SEQ ID NO:2 but has one or more of the following mutations: R38E, C53S, L54G, T57G, T57S, E59D, N80G, S83G, Y100T, K121A, D131E, D134E, E155G, T157G, T158Y, T159Y, N181D, N181A, D190A, N191A, L214A, Y221S, K226A, F235S, 1240L, N248A, D263G, G269D, R285D, D292G, H301T, K309R, K322G, R329A, L347F, L347Y, L3471, M351A, H353R, S361G, F362S, Y379S, L386A, K388G, H398A, K401R, K401D, T407S, L409G, S420T, P424A, F425S, P427A, D428G, H429A, Y430T, R431K, R431G, Y432S, Y432A, Y432F, and deletion of Tyr 432 (Δ432). Also more preferably, the mutant SAH hydrolase is a derived sequence of amino acids set forth in SEQ ID NO:1 or encoded by the sequence of nucleotides set forth in SEQ ID NO:2 and has a combination of Arg 431 to Ala (R431A) and Lys 426 to Arg (K426R) mutations. The nucleic acid molecules contemplated also include those that have conservative amino acid changes, and include those that hybridize along their full length to the coding portion of the sequence of nucleotides set forth in SEQ ID NO:2, under medium stringency, or preferably high stringency, such that the encoded protein retains ability to bind to the selected analyte without substantial conversion of the analyte.

Also provided herein is an isolated nucleic acid fragment, either DNA or RNA, that includes a sequence of nucleotides encoding a mutant S-adenosylhomocysteine (SAH) hydrolase, the mutant SAH hydrolase substantially retains its binding affinity or has enhanced binding affinity for homocysteine Hcy, SAH or adenosine but has attenuated catalytic activity.

In one specific embodiment, the isolated nucleic acid fragment encodes a mutant SAH hydrolase wherein the attenuated catalytic activity is caused by mutation(s) in the mutant SAH hydrolase's binding site for NAD+, or mutation(s) in the mutant SAH hydrolase's catalytic site or a combination thereof.

In yet another specific embodiment, the isolated nucleic acid fragment encodes a mutant SAH hydrolase wherein the mutant SAH hydrolase has a Km for SAH that is about or less than 10.0 μM. Preferably, the isolated nucleic acid fragment encodes a mutant SAH hydrolase wherein the mutant SAH hydrolase has a Km for SAH that is about 1.0 μM or less than 1.0 μM.

In yet another specific embodiment, the isolated nucleic acid fragment encodes a mutant SAH hydrolase wherein the mutant SAH hydrolase has a Kcat for SAH that is about or less than 0.1 S−1.

Further provided is a plasmid, including the nucleic acid fragment encoding the above mutant SAH hydrolases. Preferably, the plasmid is an expression vector including a sequence of nucleotides encoding: a) a promoter region; and b) a mutant S-adenosylhomocysteine (SAH) hydrolase, the mutant SAH hydrolase substantially retains its binding affinity or has enhanced binding affinity for Hcy, SAH or adenosine but has attenuated catalytic activity. The sequence of nucleotides encoding the mutant SAH hydrolase is operatively linked to the promoter, whereby the mutant SAH hydrolase is expressed. More preferably, the plasmid also includes a selectable marker.

Further provided is a recombinant host cell containing the above plasmid. The recombinant host cell can be any suitable host cell, including, but not limited to, a bacterial cell, a yeast cell, a fungal cell, a plant cell, an insect cell or an animal cell.

Also provided are methods for producing a mutant SAH hydrolase. The recombinant host cells can be grown or cultured under conditions whereby the mutant SAH hydrolase is expressed by the cell. The expressed mutant SAH hydrolase can then be isolated or recovered.

Additional mutant SAH hydrolase that substantially retains its binding affinity or has enhanced binding affinity for Hcy, SAH or adenosine, but has attenuated catalytic activity can be produced according to the procedures known to the those of skill in the art. The above-described mutant SAH hydrolases and additional mutant SAH hydrolase that substantially retain binding affinity or have enhanced binding affinity for Hcy, SAH or adenosine but have attenuated catalytic activity can be used for assaying Hcy in a sample.

Hcy Assays Using Mutant SAH Hydrolase

In one specific embodiment, the mutant Hcy-binding enzyme used in the Hcy assay is a mutant SAH hydrolase, the mutant SAH hydrolase substantially retains its binding affinity or has enhanced binding affinity for Hcy, SAH or adenosine but has attenuated catalytic activity. This assay, illustrated in detail in the EXAMPLES, is depicted in FIG. 1. In this Figure, the homocysteine-containing analyte is reduced to produce Hcy, which, is quantified or detected by binding it to a mutant (substrate trapping) SAH hydrolase; the Hcy is then converted to SAH by reaction with adenosine in the presence of wild type SAH hydrolase. As exemplified in the Figure, instead of using a monoclonal antibody to effect quantitation (see, e.g., U.S. Pat. No. 5,885,767 and U.S. Pat. No. 5,631,127). Quantitation is effected using a fluorescence-labeled tracer S-adenosylcysteine in a competition binding format in which the mutant SAH is used to trap the substrate. Any suitable quantitation assay with any suitable label can be used in the substrate trapping method. FIG. 2 depicts an exemplary assay performed in a 96 well format; and FIG. 3 exemplifies preparation of labeling of adenosyl-cysteine with a fluorescent moiety.

In one preferred embodiment, the attenuated catalytic activity in the mutant SAH hydrolase is caused by mutation(s) in the mutant SAH hydrolase's binding site for NAD+, or mutation(s) in the mutant SAH hydrolase's catalytic site or a combination thereof.

In another preferred embodiment, the mutant SAH hydrolase has attenuated 5′-hydrolytic activity but substantially retains its 3′-oxidative activity.

In still another preferred embodiment, the mutant SAH hydrolase has a Km for SAH that is about or less than 10.0 μM. More preferably, the mutant SAH hydrolase has a Km for SAH that is about 1.0 μM or less than 1.0 μM.

In yet another preferred embodiment, the mutant SAH hydrolase has a Kcat for SAH that is about or less than 0.1 S−1.

In yet another preferred embodiment, prior to the contact between the sample and the mutant SAH hydrolase, oxidized Hcy in the sample is converted into reduced Hcy. More preferably, the oxidized Hcy in the sample is converted into reduced Hcy by a reducing agent such as tri-n-butylphosphine (TBP), β-ME, DTT, dithioerythritol, thioglycolic acid, glutathione, tris(2-carboxyethyl)phosphine, sodium cyanoborohydride, NaBH4, KBH4 and free metals.

In yet another preferred embodiment, prior to the contact between the sample and the mutant SAH hydrolase, the Hcy in the sample is converted into SAH. More preferably, the Hcy in the sample is converted into SAH by a wild-type SAH hydrolase. Also more preferably, the SAH is contacted with the mutant SAH hydrolase in the presence of a SAH hydrolase catalysis inhibitor such as neplanocin A or thimersol.

In yet another preferred embodiment, prior to the contact between the SAH and the mutant SAH hydrolase, free adenosine is removed or degraded. More preferably, the free adenosine is degraded by combined effect of adenosine deaminase, purine nucleoside phosphorylase and xanthine oxidase.

Any purine nucleoside phosphorylase can be used. Preferably, the purine nucleoside phosphorylase encoded by the nucleic acids having the following GenBank accession Nos. can be used: U88529 (E. coli); U24438 (E. coli, see also Cornell and Riscoe, Biochim. Biophys. Acta, 1396(1):8–14 (1998)); U83703 (H. pylori); and M30469 (E. coli).

Any xanthine oxidase can be used. Preferably, the xanthine oxidase encoded by the nucleic acids having the following GenBank accession Nos. can be used: AF080548 (Sinorhizobium meliloti); and U39487 (Human, see also Saksela and Raivio, Biochem. J, 315(1):235–9 (1996)).

In yet another preferred embodiment, the sample containing or suspected of containing SAH is contacted with the mutant SAH hydrolase in the presence of a labeled SAH or a derivative or an analog thereof, whereby the amount of the labeled SAH bound to the mutant SAH hydrolase inversely relates to amount of the SAH in the sample. The SAH, or the derivative or analog thereof, can be labeled by methods known in the art, e.g., to become radioactive, enzymatic, fluorescent, luminescent (including chemo- or bio-luminescent) labeled. More preferably, the labeled SAH derivative or analog is a fluorescence labeled adenosyl-cysteine.

In yet another preferred embodiment, the sample containing or suspected of containing SAH is contacted with a labeled mutant SAH hydrolase. The mutant SAH hydrolase can be labeled by methods known in the art, e.g., to become radioactive, enzymatic, fluorescent, luminescent (including chemo- or bio-luminescent) labeled. More preferably, the mutant SAH hydrolase is fluorescently labeled. For example, a mutant SAH hydrolase derived from an SAH hydrolase having sequence of amino acids encoded by the sequence of nucleotides set forth in SEQ ID NO:2 is used and the mutant SAH hydrolase is fluorescently labeled at residue Cys421.

The present method can be used alone, or can be used in combination with other related assays. For example, the present method can further comprise a step of detecting cholesterol and/or folic acid in the sample, whether sequentially or simultaneously. Any suitable methods for assaying cholesterol and/or folic acid can be used. For example, the cholesterol and/or folic acid assays disclosed in WO 01/02600 can be used.

C. COMBINATIONS, KITS AND ARTICLES OF MANUFACTURE

In another aspect, the present invention is directed to a combination, which combination comprises: a) a mutant SAH hydrolase that has binding affinity for Hcy, SAH or adenosine but has attenuated catalytic activity, wherein said binding affinity and/or said attenuated catalytic activity of said SAH hydrolase is caused by a mutation in said mutant SAH hydrolase's catalytic site, its binding site for NAD+, NADH, Hcy, SAH or adenosine, or a combination thereof; and b) reagents for detecting binding between Hcy, SAH or adenosine and said SAH hydrolase. The combination can further comprise a reagent for detecting cholesterol and/or folic acid.

A kit comprising the above the combination is also provided. The kit can further comprise instructions for assaying Hcy, SAH or adenosine in a sample.

In still another aspect, the present invention is directed to an article of manufacture, which article of manufacture comprises: a) packaging material; b) a mutant SAH hydrolase that has binding affinity for Hcy, SAH or adenosine but has attenuated catalytic activity, wherein said binding affinity and/or said attenuated catalytic activity of said SAH hydrolase is caused by a mutation in said mutant SAH hydrolase's catalytic site, its binding site for NAD+, NADH, Hcy, SAH or adenosine, or a combination thereof; and c) a label indicating that the mutant SAH hydrolase and the means for use in assaying Hcy in a sample.

The packages discussed herein in relation to diagnostic systems are those customarily utilized in diagnostic systems. Such packages include glass and plastic, such as polyethylene, polypropylene and polycarbonate, bottles and vials, plastic and plastic-foil laminated envelopes and the like. The packages may also include containers appropriate for use in auto analyzers. The packages typically include instructions for performing the assays.

D. CONJUGATES

In yet another aspect, the present invention is directed to a conjugate, which conjugate comprises: a) a mutant SAH hydrolase that has binding affinity for Hcy, SAH or adenosine but has attenuated catalytic activity, wherein said binding affinity and/or said attenuated catalytic activity of said SAH hydrolase is caused by a mutation in said mutant SAH hydrolase's catalytic site, its binding site for NAD+, NADH, Hcy, SAH or adenosine, or a combination thereof; and b) a facilitating agent linked to the mutant SAH hydrolase directly or via a linker, wherein the agent facilitates: i) affinity isolation or purification of a conjugate; ii) attachment of a conjugate to a surface; or iii) detection of a conjugate. The conjugate can be a fusion protein. Alternatively, the mutant SAH hydrolase and the facilitating agent can be linked by other means.

The conjugates can be produced by chemical conjugation, such as via thiol linkages, but are preferably produced by recombinant means as fusion proteins. In the fusion protein, the peptide or fragment thereof is linked to either the N-terminus or C-terminus of the mutant enzyme. In chemical conjugates the peptide or fragment thereof may be linked anywhere that conjugation can be effected, and there may be a plurality of such peptides or fragments linked to a single mutant enzyme or to a plurality thereof.

Conjugation can be effected by any method known to those of skill in the art. As described below, conjugation can be effected by chemical means, through covalent, ionic or any other suitable linkage. For example, the reagents and methods for conjugation as disclosed in WO 01/02600 can be used.

In some embodiments, the conjugate is a fusion protein, which prior to the contact between the sample and the fusion protein, is isolated or purified. More preferably, the fusion protein is isolated or purified through affinity binding between the protein or peptide fragment of the fusion protein and an affinity binding moiety. Any kind of affinity interaction can be used for isolating or purifying the fusion protein. The affinity interactions, such as those described herein, but not limited to, are protein/protein, protein/nucleotide, protein/lipid, protein/polysaccharide, or protein/metal interactions.

In other embodiments, prior to the contact between the sample and the conjugate, such as a fusion protein, the conjugate is attached to a surface. More preferably, the conjugate is attached to the surface through affinity binding between the facilitating agent of conjugate and an affinity binding moiety on the surface. Any kind of affinity interaction can be used for attaching the conjugate, including the protein/protein, protein/nucleotide, protein/lipid, protein/polysaccharide, or protein/metal interactions.

E. EXAMPLES

The following examples are included for illustrative purposes only and are not intended to limit the scope of the invention.

Single-strand DNA-based mutagenesis was conducted based on the method described by Taylor, et al., Nucleic Acids Res., 13:8765–8785 (1985), which exploits the inability of NciI to cleave a thio-containing DNA strand. Sculptor™ invitro mutagenesis system RPN1526 (Amersham Life science, UK) was used. The pKK223-3 vector containing the wild type gene of SAH hydrolase was prepared using the method of alkaline lysis followed by plasmid purification using Promega's DNA purification kit (Wizard plus Minipreps, Promega, Madison Wis.). The purified plasmid was digested with EcoR I (Stratagene, La Jolla, Calif.) at 37° C. for 2 hours to obtain the EcoR I fragment by agarose gel electrophoresis followed by DNA purification using Promega DNA purification kit. The purified EcoR I fragment was subcloned into M13 mp19 DNA (Pharmacia Biotech, Piscataway, N.J.) by T4 DNA ligase (Pharmacia Biotech Piscataway, N.J.). The ligation was conducted in One-phor-All buffer (10 mM tris-Ac, pH 7.5, 10 mM Mg(Ac)2, 50 mM KAc; Pharmacia LKB Biotechnology AB, Uppsala, Sweden) at 4° C. overnight. The ligation product was transferred into TG1 cells (Stratagene, La Jolla, Calif.) by incubation of 10 μl of the M13 with 90 μl of competent TG 1 cells at 0° C. for 30 min. and 42° C. for 75 sec. After being chilled to 0° C. for 2 min, 500 μl of 2XYT media was added to the cells and incubated for 10 min. at 37° C. Two hundred μl of growing nontransformed TG1 cells were mixed with the transformed TG1 cells, and to which 2.5 ml of soft agarose LB (42° C.) was added. The cell mixture was immediately poured onto preheated LB agar plates (40° C.), and incubated at 37° C. overnight. Phage clones were picked up for examination of the insertion of SAH hydrolase gene and the orientation through DNA sequencing and restriction enzyme analysis. The selected phage clone was used for preparation of single strand DNA template.

The M13 phage containing the SAH hydrolase gene were incubated with TG1 cells in 3 ml of 2×YT media overnight. One drop of the overnight culture was mixed with growing TG1 cells (in log phase) in 30 ml of 2XYT media. Cells were incubated for 8 hours with shaking. After centrifugation, the supernatant was collected for single-strand template DNA purification. The purification was conducted according to the manufacture's procedure provided by Amersham Life Science.

Design of Primers for Point Mutation

Oligonucleotides (15–30 bases) were synthesized by CruaChem (Sterling, Va.). The sequence of the oligonucleotides were designed to be complementary to the sequence in the region covering both sides of the mutation site. For example, to mutate lys 426 to glu 426, the oligonucleotides used as primer contained the following sequence:

GGCCCCTTCGAGCCGGATCACTACCGC (SEQ ID NO:4) where GAG codes for glu instead of original (wild type) AAG which codes for lys.

The oligonucleotides were dissolved in water to a concentration of 5 ng/μl. The oligonucleotide solution was then phosphorylated at the 5′-end using polynucleotide kinase. The phosphorylation reaction mixture contained the following materials: 2.5 μl of oligonucleotides (5 ng/μl), 3 μl of one-phor-all 10×kinase buffer (Pharmacia Biotech), 21.5 μl of water, 2 μl of 10 mM ATP, and 1 μl of polynucleotide kinase (100,000 U/ml) (Pharmacia Biotech). The reaction mixture was incubated at 37° C. for 30 min. followed by heating at 70° C. for 10 min. to inactivate the enzyme.

TABLE 1

Oligonucleotides used for site-directed mutagenesis of human SAH

hydrolases

SEQ

Fo/

ID No.

Re

K186A

GACTTCGTCACCGCCAGCAAGTTTGGG

5

Fo

K186A

CCCAAACTTGCTGGCGGTGACGAAGTC

6

Re

F302S

AACATTGGACACTCTGACGTGGAGATC

7

Fo

F302S

GATCTCCACGTCAGAGTGTCCAATGTT

8

Re

H301D

TGTAACATTGGAGACTTTGACGTGGAG

9

Fo

H301D

CTCCACGTCAAAGTCTCCAATGTTACA

10

Re

H353S

TGTGCCATGGGCTCCCCCAGCTTCGTG

11

Fo

H353S

CACGAAGCTGGGGGAGCCCATGGCACA

12

Re

R343A

CTGGCCGAGGGTGCGCTGGTCAACCTG

13

Fo

R343A

CAGGTTGACCAGCGCACCCTCGGCCAG

14

Re

D190A

AAGAGCAAGTTTGCCAACCTCTATGGC

15

Fo

D190A

GCCATAGAGGTTGGCAAACTTGCTCTT

16

Re

F82A

AGCTGCAACATCGCCTCCACCCAGGAC

17

Fo

F82A

GTCCTGGGTGGAGGCGATGTTGCAGCT

18

Re

N181D

GCCATCAATGTGGACGACTCCGTCACC

19

Fo

N181D

GGTGACGGAGTCGTCGACATTGATGGC

20

Re

R431A

CCGGATCACTACGCCTACTGAGAATTC

21

Fo

R431A

GAATTCTCAGTAGGCGTAGTGATCCGG

22

Re

K426R

GATGGCCCCTTCCGCCCGGATCACTAC

23

Fo

K426R

GTAGTGATCCGGGCGGAAGCCATCACA

24

Re

C195S

AACCTCTATGGCTCCCGGGAGTCCCTC

25

Fo

C195S

GAGGGACTCCCGGGAGCCATAGAGGTT

26

Re

Δ432

GATCACTACCGCTGATGAGAATTCGAG

27

Fo

Δ432

CTCGAATTCTCATCAGCGGTAGTGATC

28

Re

The mutagenized codon is underlined, and the nucleotides changed are in boldface type.

Fo: forward oligonucleotide

Ro: backward oligonucleotide

TABLE 2

Additional oligonucleotides used for site-directed mutagenesis of

human SAH hydrolases

SEQ ID

Mutant

Sequence

No.

Fo/Re

Glu156Ala

GGCATCTCTGAGGCGACCACGACTGGG

29

Fo

Glu156Ala

CCCAGTCGTGGTCGCCTCAGAGATGCC

30

Re

Glu156Asp

GGCATCTCTGAGGACACCACGACTGGG

31

Fo

Glu156Asp

CCCAGTCGTGGTGTCCTCAGAGATGCC

32

Re

Asp131Lys

CTCAACATGATTCTGGACAAGGGGGGCGACCTCACC

33

Fo

Asp131Lys

GGTGAGGTCGCCCCCCTTGTCCAGAATCATGTTGAG

34

Re

Asp131Asn

CTCAACATGATTCTGGACAACGGGGGCGACCTCACC

35

Fo

Asp131Asn

GGTGAGGTCGCCCCCGTTGTCCAGAATCATGTTGAG

36

Re

Lys186Ala

GACTCCGTCACCGCGAGCAAGTTTGAC

37

Fo

Lys186Ala

GTCAAACTTGCTCGCGGTGACGGAGTC

38

Re

Lys186Asp

GACTCCGTCACCGACAGCAAGTTTGAC

39

Fo

Lys186Asp

GTCAAACTTGCTGTCGGTGACGGAGTC

40

Re

His55Pro

GCTGGCTGCCTGCCCATGACCGTGGAGACG

41

Fo

His55Pro

CGTCTCCACGGTCATGGGCAGGCAGCCAGC

42

Re

Arg343Ala

CTGCTGGCCGAGGGTGCGCTGGTCAACCTG

43

Fo

Arg343Ala

CAGGTTGACCAGCGCACCCTCGGCCAGCAG

44

Re

Asp303Glu

GTGTGTAACATTGGACACTTTGAGGTGGAGATCGATGTC

45

Fo

Asp303Glu

GACATCGATCTCCACCTCAAAGTGTCCAATGTTACACAC

46

Re

Phe302Ile

GTGTGTAACATTGGACACATTGACGTGGAGATC

47

Fo

Phe302Ile

GATCTCCACGTCAATGTGTCCAATGTTACACAC

48

Re

Leu344Gly

GCCGAGGGTCGGGGGGTCAACCTGGGTTGTGCC

49

Fo

Leu344Gly

GGCACAACCCAGGTTGACCCCCCGACCCTCGGC

50

Re

Phe82Ser

CAGTGGTCCAGCTGCAACATCTCCTCCACCCAGGAC

51

Fo

Phe82Ser

GTCCTGGGTGGAGGAGATGTTGCAGCTGGACCACTG

52

Re

Thr159Ser

GAGGAGACCACGTCCGGGGTCCACAACCTC

53

Fo

Thr159Ser

GAGGTTGTGGACCCCGGACGTGGTCTCCTC

54

Re

Asn346Gly

GGTCGGCTGGTCGGCCTGGGTTGTGCC

55

Fo

Asn346Gly

GGCACAACCCAGGCCGACCAGCCGACC

56

Re

Asn346Asp

GGTCGGCTGGTCGACCTGGGTTGTGCC

57

Fo

Asn346Asp

GGCACAACCCAGGTCGACCAGCCGACC

58

Re

Cys79Ala

GTGCAGTGGTCCAGCGCCAACATCTTCTCCACC

59

Ro

Cys79Ala

GGTGGAGAAGATGTTGGCGCTGGACCACTGCAC

60

Re

Cys79Gly

GTGCAGTGGTCCAGCGGCAACATCTTCTCCACC

61

Fo

Cys79Gly

GGTGGAGAAGATGTTGCCGCTGGACCACTGCAC

62

Re

His301Ala

GTGTGTAACATTGGAGCCTTTGACGTGGAG

63

Fo

His301Ala

CTCCACGTCAAAGGCTCCAATGTTACACAC

64

Re

Asp303Ala

GTGTGTAACATTGGACACTTTGCCGTGGAGATCGATGTC

65

Fo

Asp303Ala

GACATCGATCTCCACGGCAAAGTGTCCAATGTTACACAC

66

Re

Thr158Tyr

ATCTCTGAGGAGACCTATACTGGGGTCCACAAC

67

Fo

Thr158Tyr

GTTGTGGACCCCAGTATAGGTCTCCTCAGAGAT

68

Re

Thr159Tyr

ATCTCTGAGGAGACCACGTATGGGGTCCACAAC

69

Fo

Thr159Tyr

GTTGTGGACCCCATACGTGGTCTCCTCAGAGAT

70

Re

Fo: forward oligonucleotide

Ro: backward oligonucleotide.

TABLE 3

Additional oligonucleotides used for site-directed mutagenesis of

human SAH hydrolases

SEQ

ID

Fo/

Mutant

Sequence

No.

Re

L347F

GGTCGGCTGGTCAACTTTGGTTGTGCCATG

71

Fo

L347F

CATGGCACAACCAAAGTTGACCAGCCGACC

72

Re

R38E

ATGCGTATGCGGGAGGAATACTCGGCCTCC

73

Fo

R38E

GGAGGCCGAGTATTCCTCCCGCATACGCAT

74

Re

C53S

GCCCGCATCGCTGGCTCTCTGCACATGACC

75

Fo

C53S

GGTCATGTGCAGAGAGCCAGCGATGCGGGC

76

Re

T57G

GGCTGCCTGCACATGGGGGTGGAGACGGCC

77

Fo

T57G

GGCCGTCTCCACCCCCATGTGCAGGCAGCC

78

Re

E59D

CTGCACATGACCGTGGATACGGCCGTCCTC

79

Fo

E59D

GAGGACGGCCGTATCCACGGTCATGTGCAG

80

Re

T57S

GGCTGCCTGCACATGTCTGTGGAGACGGCC

81

Fo

T57S

GGCCGTCTCCACAGACATGTGCAGGCAGCC

82

Re

L347Y

GGTCGGCTGGTCAACTATGGTTGTGCCATG

83

Fo

L347Y

CATGGCACAACCATAGTTGACCAGCCGACC

84

Re

L347I

GGTCGGCTGGTCAACATTGGTTGTGCCATG

85

Fo

L347I

CATGGCACAACCAATGTTGACCAGCCGACC

86

Re

S83G

TGCAACATCTTCGGTACCCAGGACCATGCG

87

Fo

S83G

TGCAACATCTTCGGTACCCAGGACCATGCG

88

Re

L54G

GCCCGCATCGCTGGCTGCGGTCACATGACC

89

Fo

L54G

GGTCATGTGACCGCAGCCAGCGATGCGGGC

90

Re

Y100T

GGCATTCCGGTGACTGCCTGGAAGGGCGAA

91

Fo

Y100T

TTCGCCCTTCCAGGCAGTCACCGGAATGCC

92

Re

K121A

ACCCTGTACTTCGCTGACGGGCCCCTCAAC

93

Fo

K121A

GTTGAGGGGCCCGTCAGCGAAGTACAGGGT

94

Re

N191A

AGCAAGTTTGACGCTCTCTATGGCTGCCGG

95

Fo

N191A

CCGGCAGCCATAGAGAGCGTCAAACTTGCT

96

Re

M351A

CTGGGTTGTGCCGCTGGCCACCCCAGCTTC

97

Fo

M351A

GAAGCTGGGGTGGCCAGCGGCACAACCCAG

98

Re

H353R

CTGGGTTGTGCCATGGGCCGTCCCAGCTTC

99

Fo

H353R

GAAGCTGGGACGGCCCATGGCACAACCCAG

100

Re

F362S

TTCGTGATGAGTAACTCCAGTACCAACCAG

101

Fo

F362S

CTGGTTGGTACTGGAGTTACTCATCACGAA

102

Re

D131E

ATGATTCTGGACGAAGGGGGCGACCTCACC

103

Fo

D131E

GGTGAGGTCGCCCCCTTCGTCCAGAATCAT

104

Re

T157G

ATCTCTGAGGAGGGTACGACTGGGGTCCAC

105

Fo

T157G

GTGGACCCCAGTCGTACCCTCCTCAGAGAT

106

Re

N80G

TGGTCCAGCTGCGGTATCTTCTCCACCCAG

107

Fo

N80G

CTGGGTGGAGAAGATACCGCAGCTGGACCA

108

Re

D134E

ATGATTCTGGACGACGGGGGCGAACTCACC

109

Fo

D134E

GGTGAGTTCGCCCCCGTCGTCCAGAATCAT

110

Re

E155G

ATCTCTGGTGAGACCACGACTGGGGTCCAC

111

Fo

E155G

GTGGACCCCAGTCGTGGTCTCACCAGAGAT

112

Re

N181A

GCCATCAATGTCGCTGACTCCGTCACCAAG

113

Fo

N181A

CTTGGTGACGGAGTCAGCGACATTGATGGC

114

Re

L214A

ATGATTGCCGGCGCTGTAGCGGTGGTAGCA

115

Fo

L214A

TGCTACCACCGCTACAGCGCCGGCAATCAT

116

Re

Y221S

GTGGTAGCAGGCTCTGGTGATGTGGGCAAG

117

Fo

Y221S

CTTGCCCACATCACCAGAGCCTGCTACCAC

118

Re

K226A

GGTGATGTGGGCGCTGGCTGTGCCCAGGCC

119

Fo

K226A

GGCCTGGGCACAGCCAGCGCCCACATCACC

120

Re

F235S

GCCCTGCGGGGTTCTGGAGCCCGCGTCATC

121

Fo

F235S

GATGACGCGGGCTCCAGAACCCCGCAGGGC

122

Re

I240L

GGAGCCGCGTCCTTATCACCGAGATTGAC

123

Fo

I240L

GTCAATCTCGGTGATAAGGACGCGGGCTCC

124

Re

N248A

ATTGACCCCATCGCTGCACTGCAGGCTGCC

125

Fo

N248A

GGCAGCCTGCAGTGCAGCGATGGGGTCAAT

126

Re

D263G

GTGACCACCATGGGTGAGGCCTGTCAGGAG

127

Fo

D263G

CTCCTGACAGGCCTCACCCATGGTGGTCAC

128

Re

G269D

GAGGCCTGTCAGGAGGATAACATCTTTGTC

129

Fo

G269D

GACAAAGATGTTATCCTCCTGACAGGCCTC

130

Re

R285D

GACATCATCCTTGGCGATCACTTTGAGCAG

131

Fo

R285D

CTGCTCAAAGTGATCGCCAAGGATGATGTC

132

Re

D292G

CAGATGAAGGGTGATGCCATTGTGTGTAAC

133

Fo

D292G

GTTACACACAATGGCATCACCCTTCATCTG

134

Re

H301T

AACATTGGAACTTTTGACGTGGAGATCGAT

135

Fo

H301T

ATCGATCTCCACGTCAAAAGTTCCAATGTT

136

Re

K309R

GAGATCGATGTCAGATGGCTCAACGAGAAC

137

Fo

K309R

GTTCTCGTTGAGCCATCTGACATCGATCTC

138

Re

K322G

GTGAACATCGGTCCGCAGGTGGACCGGTAT

139

Fo

K322G

ATACCGGTCCACCTGCGGACCGATGTTCAC

140

Re

R329A

GACCGGTATGCTTTGAAGAATGGGCGCCGC

141

Fo

R329A

GCGGCGCCCATTCTTCAAAGCATACCGGTC

142

Re

S361G

ATGAGTAACGGTTTCACCAACCAGGTGATG

143

Fo

S361G

CATCACCTGGTTGGTGAAACCGTTACTCAT

144

Re

Y379S

CATCCAGACAAGTCTCCCGTTGGGGTTCAT

145

Fo

Y379S

ATGAACCCCAACGGGAGACTTGTCTGGATG

146

Re

L386A

GGGGTTCATTTCGCTCCCAAGAAGCTGGAT

147

Fo

L386A

ATCCAGCTTCTTGGGAGCGAAATGAACCCC

148

Re

K388G

CATTTCCTGCCCGGTAAGCTGGATGAGGCA

149

Fo

K388G

TGCCTCATCCAGCTTACCGGGCAGGAAATG

150

Re

H398A

GCAGTGGCTGAAGCCGCTCTGGGCAAGCTG

151

Fo

H398A

CAGCTTGCCCAGAGCGGCTTCAGCCACTGC

152

Re

K401R

CACCTGGGCCGTCTGAATGTGAAGTTGACC

153

Fo

K401R

GGTCAACTTCACATTCAGACGGCCCAGGTG

154

Re

K401D

CACCTGGGCGATCTGAATGTGAAGTTGACC

155

Fo

K401D

GGTCAACTTCACATTCAGATCGCCCAGGTG

156

Re

T407S

AATGTGAAGTTGTCTAAGCTAACTGAGAAG

157

Fo

T407S

CTTCTCAGTTAGCTTAGACAACTTCACATT

158

Re

L409G

GTGAAGTTGACCAAGGGTACTGAGAAGCAA

159

Fo

L409G

TTGCTTCTCAGTACCCTTGGTCAACTTCAC

160

Re

S420T

TACCTGGGCATGACTTGTGATGGCCCCTTC

161

Fo

S420T

GAAGGGGCCATCACAAGTCATGCCCAGGTA

162

Re

P424A

TCCTGTGATGGCGCTTTCAAGCCGGATCAC

163

Fo

P424A

GTGATCCGGCTTGAAAGCGCCATCACAGGA

164

Re

F425S

TGTGATGGCCCCTCTAAGCCGGATCACTAC

165

Fo

F425S

GTAGTGATCCGGCTTAGAGGGGCCATCACA

166

Re

D428G

TGTGATGGCCCCTTCAAGCCGGGTCACTAC

167

Fo

D428G

GTAGTGACCCGGCTTGAAGGGGCCATCACA

168

Re

P427A

TGTGATGGCCCCTTCAAGGCTGATCACTAC

169

Fo

P427A

GTAGTGATCAGCCTTGAAGGGGCCATCACA

170

Re

H429A

TGTGATGGCCCCTTCAAGCCGGATGCTTAC

171

Fo

H429A

GTAAGCATCCGGCTTGAAGGGGCCATCACA

172

Re

Y430T

GGCCCCTTCAAGCCGGATCACACTCGCTAC

173

Fo

Y430T

GTAGCGAGTGTGATCCGGCTTGAAGGGGCC

174

Re

R431K

GGCCCCTTCAAGCCGGATCACTACAAATAC

175

Fo

R431K

GTATTTGTAGTGATCCGGCTTGAAGGGGCC

176

Re

R431G

GGCCCCTTCAAGCCGGATCACTACGGTTAC

177

Fo

R431G

GTAACCGTAGTGATCCGGCTTGAAGGGGCC

178

Re

Y432S

CCCTTCAAGCCGGATCACTACCGCTCTTGA

179

Fo

Y432S

TCAAGAGCGGTAGTGATCCGGCTTGAAGGG

180

Re

Y432A

CCCTTCAAGCCGGATCACTACCGCGCTTGA

181

Fo

Y432A

TCAAGCGCGGTAGTGATCCGGCTTGAAGGG

182

Re

Y432F

CCCTTCAAGCCGGATCACTACCGCTTTTGA

183

Fo

Y432F

TCAAAAGCGGTAGTGATCCGGCTTGAAGGG

184

Re

Fo: forward oligonucleotide

Re: backward oligonucleotide

The 5′-phosphorylated oligonucleotides DNA was annealed with single-stranded DNA (M13 phage containing wild type human SAH hydrolase gene, 1 μg/l) in a ratio of oligonucleotide: template of 2:1 in annealing buffer. The annealing reaction was performed by incubating the annealing mixture at 70° C. for 3 min. followed by 30 min. at 37° C. or followed by transferring the micro centrifuge tube to a 55° C. beaker and then allowed to cool to room temperature. To the annealing mixture (17 μl), 19 μl of dNTP A (α-S) mix, 1.5 μl of T7 DNA polymerase (0.8 units), and 2.5 μl of T4 DNA ligase (92.5 units), and 6 μl of water were added. After 10 min. at room temperature and 30 min. at 37° C., the reaction was stopped by heat inactivation at 70° C. for 15 min. To the reaction mixture was added T5 exonuclease (2000 units) and exonuclease buffer to remove single-strand non-mutant DNA at 37° C. for 30 min. followed by 15 min. of heat inactivation at 70° C. NciI (5 units) was added to the reaction mixture to nicking the non-mutant strand by incubating NciI at 37° C. for 90 min. The non-mutant strand was digested by adding 160 units of Exonuclease III and incubating at 37° C. for 30 min. followed by heat inactivation. To repolymerize the gaped DNA, dNTP mix B and 3.5 units of DNA polymerase 1 and 2.5 units of T4 DNA ligase were added to the reaction mixture, and incubated at 37° C. for 1 h.

The M13 plasmid containing the mutated SAH hydrolase gene was then transferred into competent TG 1 host cells by heat shock method or an electroporation method. Ten μl of the mutant M13 plasmid was added to 90 μl of water and mixed with competent TG1 cells in ice for 40 min. The TG1 cells were shocked by incubation at 42° C. for 45 sec. and immediately at 0° C. for 5 min. The transferred TG1 cells were allowed to return to room temperature, and mixed with 200 μl of growing non-transferred TG1 cells (served as lawn cells). Three ml of molten Htop agar was added and mixed followed by immediately pouring the cells onto a L plate. The plate was incubated in 37° C. for overnight. Phage plaques formed were picked by sterile tooth pick and swirling in a tube containing 3 ml of 2XYT medium. After overnight culture, cells were collected by centrifugation, and the double-strand M13 plasmid from the cells was purified by using Promega DNA purification kit (Wizard plus Minipreps).

The supernatant from centrifugation was used to purify single-strand M13 DNA. The mutation was confirmed by DNA sequencing of the single-strand M13 DNA using Sequenase Version 2.0 (Unites States Biochemical). The double-strand M13 DNA containing correct mutation sequence was selected, and digested with EcoR I. The EcoR I fragment containing the mutant SAH hydrolase gene was purified by agarose electrophoresis followed by gene cleaning using Qlaquick Gel Extraction kit (Qiagen, Valencia, Calif.). The purified EcoR I fragment was subcloned into pKK223-3 expression vector using T4 ligase. Two μl of EcoR 1 treated and 5′-dephosphorylated pKK223-3 vector backbone was incubated with 10 μl of the purified mutant insert DNA in a backbone to insert ratio of 2:1. The ligation reaction was carried out in One-phore-All buffer containing 0.01 M ATP at 16C overnight. The ligated vector containing mutant SAH hydrolase gene was transferred into competent E. Coli JM109 cells by heat shock method. The transformed cells were selected against 100 μl/ml ampicillin. Ampicillin-resistant clones were picked and grown in 10 ml of 2×YT medium containing 35 μl/ml ampicillin for 2 hours at 37° C. and then induced with 1 mM isopropyl-1-thio-β-D-galactopyranoside (IPTG) and grown overnight at 37° C. The cells were harvested by centrifugation, and suspended in 0.8 ml of 50 mM Tri-HCl, pH 7.5, containing 2 mM EDTA. Cells were lysed by rapid freezing and thawing. After centrifugation at 13,500 rpm for 1 hour at 4° C., the supernatant was collected for SDS-PAGE analysis for over-expression of SAH hydrolase mutant protein. A heavy protein band at molecular size of 47,000 Da indicates the overexpression of mutant SAH hydrolase protein.

PCR-Based Mutagenesis Method

PCR-based mutagenesis was performed using the ExSite PCR-based Site-Directed Mutagenesis Kit (Stratagene, La Jolla, Calif.). The ExSite method uses increased template concentration and <10 PCR cycles. The resulting mixture of template DNA, newly synthesized DNA and hybrid parental/newly synthesized DNA is treated with Dpn I and Pfu DNA polymerase. Dpn I digests the in vivo methylated parental template and hybrid DNA, and Pfu DNA polymerase polishes the ends to create a blunt-ended PCR product. The end-polished PCR product is then intramolecularly ligated together and transformed into E. coli cells. The detailed experimental procedure is described as follows:

To a microcentrifuge tube were added 0.5 pmol of template DNA, 2.5 μl of 10×mutagenesis buffers, 1 μl of 25 mM dNTP mix, 15 pmol of each primer, and ddH2O to a final volume of 24 μl. To the reaction mixture was then added 1 μl of ExSite DNA polymerase blend (5 U/μl). The reaction solution was overlayed with 20 μl of mineral oil and thermal cycle the DNA using 7012 amplification cycles. The cycling parameters are listed in Table 10.

TABLE 3

Mutagenesis Cycling Parameters

Segment

Cycles

Temperature

Time

1

1

94° C.

4 min.

50° C.

2 min.

72° C.

2 min.

2

8

94° C.

1 min.

56° C.

2 min.

72° C.

1 min.

72° C.

5 min.

3

72° C.

5 min.

Following amplification, the reaction tube was placed on ice for 2 min. to cool the reaction to <37° C. To the reaction tube were added 1 μl of the Dpn I restriction enzyme (10 U/μl) and 0.5 μl of cloned Pfu DNA polymerase (2.5 U/μl) followed by incubation at 37° C. for 30 min. The reaction was stopped by heating at 72° C. for 30 min. For ligating the product, to the reaction tube were added 100 μl of ddH2O, 10 μl of 10×mutagenesis buffer, and 5 μl of 10 mM rATP. Transfer 10 μl of the above reaction mixture to a new micocentrifuge tube and add 1 μl of T4 DNA ligase (4 U/μl). The ligation was incubated at 37° C. for 1 hour. 2 μl of the ligated DNA was added to 80 μl of Epicurian Coli XL1-Blue supercompetent cells on ice and incubated for 30 min. followed by 45 seconds at 42defendant and 2 min. on ice. The transformed cells were immediately plated on LB-ampicillin agar plates which had been spread with 20 μl of 10% X-gal prepared in DMF and 20 μl of 100 M IPTG in H2O. The plate was incubated overnight at 37° C. The blue colonies were selected as colonies containing the mutagenized plasmid. The selected colonies were further confirmed by DNA sequencing. Protein overexpression and substrate trapping screening were performed as described above.

Deletion and insertion mutations were also performed according to the manufacture's protocol using ExSite PCR-based Site-directed Mutagenesis Kit. Double mutations or combinations of mutation and deletion or insertion were carried out using mutated or deleted DNA as template for secondary mutation or deletion using either M13-based mutagenesis or PCR-based mutagenesis methods.

Identification of Substrate Trapping SAH Hydrolase

The cell-free extracts from colonies that inducibly overexpressed mutant SAH hydrolase proteins were chromatographed on a monoQ column (HR5/5) equipped with FPLC system. Proteins were eluted with a linear gradient of NaCl from 0 to 1 M in 10 mM sodium phosphate buffer, pH 7.0 over 35 min. The major protein peak that eluted at the same or close retention time as that of the wild type SAH hydrolase was collected. An aliquot collected mutant SAH hydrolase (1–10 μg) was incubated with [3H]SAH (10 mCi/mmole, 200 μM) and 30 μM of 5,5′-dithiobis (2-nitrobenzoic acid) (DTNB) at room temperature for 5–30 min.

The reaction solution was filtered through a membrane of molecular weight cut-off at 30,000 by centrifugation. The filtrate was measured at 412 nm for Hcy formation (enzyme activity) and the [3H] radioactivity on the membrane was measured by scintillation counting after membrane washing with 1 ml of 50 mM phosphate buffer, pH 7.0.

The mutant hydrolases that show high radioactivity on the membrane and low O.D. at 412 nm of the filtrate relative to the wild type enzyme were selected as candidates for further characterization including determination of Km or Kd and binding energy (AG). Mutant SAH hydrolases with Km value lower than 10 μM toward SAH and kcat value lower than 0.1 per second were overexpressed in larger quantity (1–2 L of E. coli culture) and the enzyme proteins were purified to homogenous as judged by single band on SDA-PAGE.

The cell-free extract of IPTG-induced E. Coli JM109 (containing SAH hydrolase gene in pKK223-3 vector) culture was mixed with powder DEAE-cellulose (Sigma, St. Louis, Mo.) equilibrated with 0.1 M sodium phosphate buffer, pH 7.2 containing 1 mM EDTA (buffer A). The cell-free extract and DEAC-cellulose mixture was placed in a funnel and filtrated under vacuum. After washing with 3 volumes of buffer A, the filtrate was precipitated by solid ammonium sulfate (30–60%). The precipitated protein was collected by centrifugation at 13000 rpm, and resuspended in 50 mM sodium phosphate buffer, pH 7.2, containing 1 mM EDTA. The protein was chromatographed through a Sephacryl S-300 size exclusion column (2.5×100 cm) (Pharmacial Biotech, Piscataway, N.J.) followed by a DEAE-Sepharose ion exchange column (2.5×30 cm) eluted by a linear NaCl gradient. The major protein peak from DEAE-Sepharose was examined by SDS-PAGE. In most of the times, this purification procedure gave a single protein band on SDS-PAGE. Sometime, minor bands were observed on SDS-PAGE. In this case, rechromatography on DEAE-Sepharose column was performed to obtain pure protein. SAH hydrolase activity or [3H]SAH binding affinity was also measured to confirm the protein peak.

Storage of the Purified SAH Hydrolase

The purified wild type and mutant SAH hydrolases were dialyzed against 5 mM sodium phosphate buffer, pH 7.0 for 6 hours at 4° C. The protein was then frozen in liquid nitrogen and lyophilized under vacuum. The lyophilized protein was stored at −70° C. The protein was stable for at least 2 years. The purified protein can also be stored in liquid containing 20% of glycerol at −20° C. For wild type enzyme, addition of 5 mole excess of adenosine (Ado) to the 20% glycerol solution stabilizes the enzyme activity even better.

Assays for Enzyme Activity

The assay of SAH hydrolase activity in the hydrolytic direction was performed as described in Yuan, et al., J. Biol. Chem., 271:28008–28016, 1996). The assay measures the hydrolysis of SAH into Ado and Hcy. The reaction product Hcy was derivatized by thiol specific reagent DTNB for colometric determination at 412 nm. The assay for SAH hydrolase in the synthetic direction was measured by the formation of SAH from substrate Ado and Hcy using HPLC (see, Yuan, et al., J. Biol. Chem., 268:17030–17037 (1993). One unit of the enzyme activity was defined as the amount of enzyme that can hydrolyze or synthesize 1 μ mole of SAH/min/mg.

Assay for Binding Affinity (Kd)

For mutant enzyme that completely lacks activity, the binding constant (Kd) values were determined by an equilibrium dialysis technique using [3H] SAH and Spectrum 5-cell Equilibrium Dialyzer) (Spectrum, Houston, Tex.). The membrane disc used had molecular cut-off of 25,000. Kd values for several exemplary SAH hydrolase mutant enzymes are shown in FIG. 5.

Example 3

Preparation of Reagents

Preparation of Fluorophore-Labeled Ado and SAH Analogs

Method 1

Ado-5′-carboxylic acid (Sigma, St. Louis, Mo.) was derivatized with 9-(hydroxylmethyl)anthracene (HMA) (Fluka, Buchs, Switzerland). To 10 mg of Ado-5′-carboxylic acid dissolved in 100 ml of chloroform (10 min sonication) was added 50 mg 1-hydroxybenzotriazole (HOBT) (Janssen Chimica, Beerse, Belgium). After evaporation to dryness under nitrogen, 300 mg of N-ethyl-N′-(3-dimethylaminopropyl)carbodiimide hydrochloride in 300 ml chloroform and 5 ml of triethylamine were added. The resulting solution was kept at 0° C. for 30 min. To the above reaction mixture was added 200 mg HMA in 100 ml of chloroform. The mixture was allowed to stand at room temperature for 10 min. and then evaporated to dryness under a stream of nitrogen. The residue obtained was dissolved in 10 ml of HPLC mobile phase (methanol-water mixture, 90:10, w/w). One ml of the above solution was injected into a semi-preparative column (Econosphere, C18, 7×300 mm, Alltech, Dearfield, Ill.). The column was eluted with an isocratic method. The flow rate was 2 ml/min. The peaks were monitored at UV260 nm and fluorescence at Ex-365 nm, Em-415 nm. The peaks with UV and fluorescence absorbance were collected as HMA-labeled Ado-5′-ester.

Method 2

Ado-5′caroboxylic acid and 4-bromomethyl-7-methoxycoumarin (Br-Mmc) (Sigma, St. Louis, Mo.) were dissolved in ethyl acetate in a molar ratio of 1:3. The reaction volume was 25 ml. After addition of 2 g of finely powdered K2CO3 the solution was refluxed for 1 hour using a ml-reluxer. After cooling, the reaction solution was injected into a C18 column (Econosphere, C18, 7×300 mm, Alltech, Deerfield, Ill.) for HPLC separation. The elution was monitored by UV (260 nm) and fluorescence (Em 328 nm and Ex390 nm). The elution was performed in a linear gradient of methanol:water from 20 to 100% over 30 min. The flow rate was 2 ml/min.

Method 3

This method is depicted in FIG. 3. Adenosyl-L-cysteine (Ado-Cys) and 4-Bromomethyl-7-methoxycoumarin (Br-Mmc) were dissolved in ethyl acetate in a molar ration of 1:3. The final volume was 25 ml (ca, 1 mg Ado-Cys). After addition of 200 mg of finely powdered K2CO3, the solution was refluxed for 1 hour using a ml-refluxer at 80° C. After cooling, the reaction solution was injected into a C18 column (Econosphere, C18, 7×300 mm, Alltech, Dearfield, Ill.) for separation using HPLC. The fluorescently labeled Ado-Cys was eluted by a linear gradient of methanol; water from 20 to 100% in 30 min. The flow rate was 2 ml/min.

Method 4

Ado-Cys was dissolved in carbonate buffer, pH 9.0 in 1 mM concentration. Fluorescein isotiocyanate (FITC) (PcPierce, Rockford, Ill.) was dissolved in DMSO in 5 mM concentration, and diluted to 1 mM with carbonate buffer, pH 9.0. Equal volumes of Ado-Cys and FITC in carbonate buffer were mixed and incubated in room temperature for 1 hour. The Ado-Cys-FITC conjugate was then isolated by HPLC using a C18 column (Econsphere, C18, Alltech, Deerfield, Ill.). The elution was monitored at UV 260 nm and fluorescence at Ex484 nm and Em520 nm. The mobile phases were water and methanol in a linear gradient from 0 to 80% of methanol in 35 min.

Coating Mutant SAH Hydrolase on Microtiter Well (96 Well Plate)

Mutant SAH hydrolase (F302S) was coated on flat-bottomed 96 well plate (Dynex Technologies, Chantilly, Va.). 200 μl of 20 μg/ml of F302S mutant hydrolase in 50 mM sodium phosphate buffer, pH 7.6. was added to each well. After incubation at 4° C. overnight, the plate was emptied by inversion. After blocking with 0.5% BSA, the plate was then washed three times with 10 mM PBS containing 0.1 NaCl and 0.05% of Tween 20. After inversion and tapping, the plate was stored at 4° C. before use.

Preparation of Standard Samples and Chemical Reagents

1. Construction of a Standard Hcy Curve

Human albumin (Fraction V powder, Sigma) was dissolved in PBS in a protein concentration equal to that of human plasma. To 10 ml of the albumin was added 4 ml of 1% tri-n-butylphosphine (TBP). The mixture was incubated at room temperature for 15 min. followed by gel filtration through a size exclusion column (Sephacryl-S100, 2×90 cm). The albumin protein concentration was normalized to human plasma concentration using protein concentrator (Bio-Rad). The protein concentration was determined by Bradford reagent (Bio-Rad). A series of known concentration of L-homocysteine and L-homocystine were spiked into the TBP-treated human albumin in a final concentrations ranging from 0 to 50 μM. After incubation at 37° C. for 1 hour, the L-homocysteine spiked albumin and L-homocystine albumin were aliquoted in 70 μl/tube as standard samples, and stored at −20° C. before use.

Tri-n-butylphoshine (Sigma) was dissolved in dimethylformamide (DMF) to 1% concentration.

4. Fluorophore-Labeled Ado-Cys Solution

Br-Mmc-labeled Ado-Cys or FITC-labeled Ado-Cys was dissolved in 50 mM phosphate buffer, pH 7.2, in a concentration of 0.5 mM.

5. SAH Hydrolase Inhibitor Solution

Neplanocin A (natural product), an inhibitor of SAH hydrolase, and a substrate of adenosine deaminase, was dissolved in 50 mM phosphate buffer, pH 7.2. The inhibitor solution (50 μM) was used in an enzyme to inhibitor ratio of 1:1.5.

To 50 μl of plasma sample in microcentrifuge tube or in uncoated 96-well plate was added 20 μl of 1% TBP and 50 μl of the wild type SAH hydrolase solution. After incubation at 25° C. for 15 min, 20 μl of the enzyme inhibitor solution was added to the reaction mixture, and incubated for 10 min. to inactivate SAH hydrolase.

Step 2. Removal of Remaining Ado and Enzyme Inhibitor

To the solution in Step 1 was added 30 μl of the multi-enzyme solution, and incubated for 15 min at room temperature.

Step 3. Trapping the Formed SAH Onto the Mutant SAH Hydrolase

150 μl solution in Step 2 was transferred to a microtiter well pre-coated with mutant SAH hydrolase. After 30 min. incubation at room temperature, the solution was emptied by inversion.

Step 4. Washing

The plate from Step 3 was washed three times with the washing solution followed by inversion and tapping.

Step 5. Binding of Fluorophore-Labeled Ado-Cys to the Mutant Enzyme

100 μl of the fluorophore-labeled Ado-Cys or fluorophore-labeled Ado-5′ ester was added to the microtiter well in Step 4. After 20 min. incubation at room temperature, the plate was washed three times with the washing solution.

To the microtiter well from Step 5, 200 μl of 50 mM phosphate buffer, pH 7.2, was added, and the plate was read for fluorescence using a plate reader (Molecular Devices, fmax). The plasma Hcy concentration was calculated from the standard curve constructed under the same conditions.

Alternative Hcy Assay

Alternatively, the Hcy assay can also be performed by pre-coating SAH on microtiter well, and using fluorophore-labeled mutant SAH hydrolase for competition binding assay. The details are described as follows:

1. Pre-Coating SAH on Microtiter Well

SAH was conjugated to polylysine by activating the carboxylic group of SAH with PCl3 at 50° C. The SAH-polylysine conjugate was purified by HPLC, and dissolved in 0.1 M carbonate buffer, pH 9.6. 300 μl of 100 μg/ml SAH-polylysine solution was added to each well, and incubated at 37° C. for 6 hours. The plate was then washed three times with washing solution containing 10 mM PBS, 0.1 M NaCl and 0.05% Tween 20. After inversion and tapping, the plate was stored at 4° C. before use.

2. Fluorophore-Labeled Mutant SAH Hydrolase

Mutant SAH hydrolase (e.g., F302S) was specifically fluorescence labels on Cys421, an non-essential cysteine residue which is located on the surface of the protein that is not involved in substrate binding and catalysis. Cys421 residue is readily accessible by thiol reactive molecules, and can be modified without effecting the binding affinity of the enzyme. Thiol specific reactive probes such as 7-diethylamino-3(4′-maleimidylphenyl)-4-methylcoumarin (CPM) can specifically label protein thiols. Mutant SAH hydrolase (F302S) (0.5 mg/ml) in 50 mM phosphate buffer, pH 7.2, was incubated with 2 mM of adenine to protect other thiols in the substrate binding site, followed by addition of CPM to final concentration of 50 μM. The reaction mixture was incubated at room temperature for 30 min. followed by gel filtration on a size exclusion column (Sephacryl S-300, 4.5 mm×60 cm) to remove adenine and excess CPM. The CPM-labeled F302S mutant SAH hydrolase (2 mg/ml) was kept in 50 mM phosphate buffer containing 20% glycerol at −20° C. The comparison of Km (SAH) and Kcat (SAH) for wild type and mutant F302S is shown below in Table 11.

TABLE 4

Comparison of kinetic constants between mutant and wild type SAH

hydrolases

Enzyme

Km (SAH)

Kcat (SAH)

wild type

7.9 μM

3.8 S−1

F302S

1.0 μM

0.1 S−1

Plasma Hcy Assay Procedure 2

Step 1. Conversion of Hcy to SAH

To 50 μl of plasma sample in microcentrifuge tube or in uncoated 96-well plate was added 20 μl of 1% TBP and 50 μl of the wild type SAH hydrolase solution. After incubation at 25° C. for 15 min, 20 μl of the enzyme inhibitor solution was added to the reaction mixture, and incubated for 10 min. to inactivate SAH hydrolase.

Step 2. Removal of Remaining Ado and Enzyme Inhibitor

To the solution in Step 1 was added 30 μl of the multi-enzyme solution, and incubated for 15 min. at room temperature.

Step 3. Competition Binding of SAH to the Mutant SAH Hydrolase

One hundred μl of the solution from Step 2 was transferred to a microtiter well pre-coated with polylysine-SAH conjugate to which 150 μl of the fluorophore-labeled mutant SAH hydrolase was added. After incubation at room temperature for 30 min., the plate was inverted and tapped followed by three times of washing with the washing solution.

Step 4. Detection of the Fluorophore-Labeled Mutant SAH Hydrolase Bound to the Microtiter Well

To the plate from Step 3 was added 200 μl of 10 nM PBS, and the plate was read by a plate reader (Molecular Devices, fmax) at Ex390 nm and Em460 nm. The plasma concentration of Hcy was calculated from the standard curve constructed under the same conditions with the standard samples.

The above examples are included for illustrative purposes only and are not intended to limit the scope of the invention. Many variations to those described above are possible. Since modifications and variations to the examples described above will be apparent to those of skill in this art, it is intended that this invention be limited only by the scope of the appended claims.